Printed circuits are formed with major dimensions of length and width and contain one or more circuits. The thickness of the printed circuit panels varies for many different reasons and directly affects panel flexibility.
For example, printed circuits made with multiple conductor planes use a bonded dielectric layer or layers for separation. Holes perforated through the circuit substrate serve a number of purposes including solder terminals for installation into another assembly, plated through hole interconnections between copper conductors, and tooling registration holes. At one stage, some or all of the copper conductors must be coated with solder, often referred to as solder leveling the circuits.
After being soldered, for example in a solder leveling apparatus described in applicant's U.S. Pat. No. 4,608,941 for Apparatus For Soldering Printed Circuit Panels, the printed circuit panels must be cleaned. Specifically, flux and other impurities remaining on the printed circuits must be removed.
In addition, acid or alkaline based cleaners are used as etchants in processing printed circuits to remove copper oxides or to micro-etch the copper itself. These substances also must be removed by cleaning the printed circuits after such process steps.
In the past, printed circuits have been cleaned by conveying them past rotating brushes used with water or slurries as shown, for example, in U.S. Pat. No. 3,928,064 and U.S. Pat. No. 4,383,494. In addition to requiring cleaning stations of substantial length, such apparatus does not effectively remove all surface contaminants nor does it clean rapidly moving printed circuit panels. Moreover, fragile printed circuits are subject to deterioration and even breakage by the brushes.
Another commonly used method of cleaning printed circuit panels is by direct immersion in hot water containing detergents. The cleaning and rinsing process can be accelerated by agitation. However, this cleaning method does not lend itself to continuous cleaning of printed circuits being moved rapidly by a conveyor.
Other apparatus presently being used for cleaning printed circuits involves the use of spray washing and rinsing. Distribution manifolds containing multiple spray nozzles are directed towards the circuit panel surfaces. Since manifolds are high pressure, low velocity conduits, the nozzles create a pressure drop that propels the cleaning liquids at a high velocity to the printed circuit surfaces. The impact of the liquids results in an energy transfer that dislodges contaminates from the surface.
To maintain continuous effective spray cleaning at desired conveyor speeds, drainage of surface liquids is important. A printed circuit panel positioned in a horizontal plane has no gravitational drainage forces. Thus surface standing water (puddling) absorbs some of the impact energy which helps drainage but diminishes cleaning power. For this reason etching systems have different stock removal rates between top and bottom surfaces of printed circuits.
Examples of spray nozzle cleaning systems include U.S. Pat. No. 3,905,827, U.S. Pat. No. 3,421,211, U.S. Pat. No. 3,868,272, and U.S. Pat. No. 4,479,849.
It will be apparent from a review of these patents that the results obtained from using spray nozzles are affected by the number of nozzles, their distance from the surface, the nozzle spray angle, the nozzle orifice size, the manifold pressure used, the spray shape (fan, cone, etc.), fixed or oscillating manifolds, and the time the printed circuits are exposed to the spray nozzles which may depend on conveyor speed. Moreover, the cleaning apparatus requires process stations that use ten, fifteen or more lineal feet.
None of the prior art spray nozzle systems has proven satisfactory for effectively cleaning printed circuits traveling at conveyor speeds now used when printed circuits are processed, i.e., etched or soldered with state of the art processes.